Injured neurons in the medicinal leech can accurately regenerate functional connections with their normal targets, thus restoring at the level of single cells the wiring established during development. The proposed project addresses the questions of how the neruon locates its specific synaptic targets, what triggers the axon to start and stop growing, what roles are played by the surrounding glia, neighboring neurons and target neurons, how the new synapses compare with the old in structure, distribution and function, and whether regeneration revives certain mechanisms that operate during development. The methodology will include extracellular and intracellular electrophysiological recording and intracellular injection of markers for subsequent light- and electron microscopic examination. The morphology of certain cells and the complex patterns of normal and regenerated synaptic contacts between them will be reconstructed with a computer. Regeneration will be studied in culture as well as within the leech; single cells can be killed with intracellular injection of proteases or of dyes that produce photodynamic damage. Growth of individual neurons will be studied during development and compared with regeneration. Monoamine-containing neurons can be histochemically induced to fluoresce. This class of neuron in mammals is unusual in its ability to regenerate; in the leech the loss and reappearance of fluorescence can be correlated with degeneration and growth of single identified neurons. Also under study will be one means of particularly rapid repair in the leech that occurs when an injured neuron forms an electrical synapse on its severed axon, which then temporarily serves as a splice until regeneration is complete. The severed axon segment may also guide the growing neuron to its target. An understanding of the mechanisms for accurate regeneration in the leech central nervous system might suggest ways that nerve regeneration in higher animals could be made reliable and precise.